1. Field of the Invention
The present invention relates to a small dual frequency band hybrid mode feed and, more particularly, to a small dual frequency band hybrid mode feed comprising a helical winding of wire supported on the inside of a hollow conical waveguide structure by a layer of dielectric material with a periodic tuned grid structure embedded in the dielectric layer.
2. Description of the Prior Art
Typical communication systems operate with two separated frequency bands of interest for transmit and receive. For radio systems, feedhorns are used to launch the electromagnetic energy for reception by a distant receiver. The corrugated horn antenna is a well known device which provides a circularly symmetrical radiation pattern which is essentially free of primary sidelobes. The corrugated horn antenna is, however, generally bandwidth limited and various arrangements have been conceived to broaden the bandwidth of the corrugated horn antenna. For example, in U.S. Pat. No. 4,040,061 issued to C. G. Roberts et al on Aug. 2, 1977, a horn antenna with broadband corrugations is provided by the addition of a dissiptative TM.sub.11 mode suppressor means disposed in the input waveguide feed to the horn. In another arrangement disclosed in U.S. Pat. No. 4,021,814 issued to J. L. Kerr et al on May 3, 1977 a broadband corrugated horn antenna is provided by providing a corrugated ridge pattern with gaps therebetween in which the width of the gaps is greater than the width of the ridges.
Hybrid mode feeds have also been designed which have the advantage over standard TE.sub.10 -rectangular and TE.sub.11 -circular mode feeds of radiating a symmetrical pattern about their axis so that the main beam contours would be identical for both polarizations. Typical feeds with mode converters included therein are shown in, for example, U.S. Pat. No. 3,618,106 issued to G. H. Bryant on Nov. 2, 1971 wherein the TE.sub.11 -circular mode is converted to the HE.sub.11 -circular mode by the use of corrugations which decrease in depth in the direction of propagation. Another example of such mode conversion is disclosed in the article "Characteristics of a Broadband Microwave Corrugated Feed: A Comparison Between Theory and Experiment" by C. Dragone in BSTJ, Vol. 56, No. 6, July-August 1977 at pp. 869-888.
Hybrid mode feeds which convert the TE.sub.11 -circular mode the the HE.sub.11 -circular mode using helically wound wire structures of various configurations that are bonded to the interior surface of a waveguide are disclosed in U.S. Pat. Nos. 4,231,042 issued to R. H. Turrin on Oct. 28, 1980 and 4,246,584 issued to A. R. Noerpel on Jan. 20, 1981.
The problem remaining in the prior art is to provide a feed which will function for two arbitrarily separated frequency bands, be capable of being built with thin walls and a small aperture to allow for use as an array element of a closely spaced phased array and also radiate a far field power pattern with main lobe contours identical for two orthogonal polarizations.